[BUG FIX] Call push_freq_mgr_.next() for next push period

Summary:
The bug is that the next sampling time returns the next push time.
Because push time were never updated, the sampling time will be always 0, which means the iteration
will be repeating without any sleep, thus increases cpu usage.

Test Plan: Jenkins

Reviewers: #stirling, oazizi

Reviewed By: #stirling, oazizi

Differential Revision: https://phab.corp.pixielabs.ai/D8916

GitOrigin-RevId: fb11e0b

src/stirling/core/utils.h

@@ -89,7 +89,7 @@ class SamplePushFrequencyManager {
    * @return std::chrono::milliseconds
    */
   px::chrono::coarse_steady_clock::time_point NextSamplingTime() const {
-    return push_freq_mgr_.next();
+    return sampling_freq_mgr_.next();
   }
 
   /**

src/stirling/core/utils_test.cc

@@ -49,5 +49,33 @@ TEST(FrequencyManagerTest, SetPeriodEndAndCheck) {
   EXPECT_GE(computed_period, std::chrono::milliseconds{9990});
 }
 
+// Tests that the sampling period were set correctly for SamplePushFrequencyManager.
+TEST(SamplePushFrequencyManagerTest, SetSamplingPeriodEndAndCheck) {
+  SamplePushFrequencyManager mgr;
+  mgr.set_sampling_period(std::chrono::milliseconds{10000});
+  EXPECT_EQ(mgr.sampling_period(), std::chrono::milliseconds{10000});
+  EXPECT_TRUE(mgr.SamplingRequired());
+
+  mgr.Sample();
+  EXPECT_FALSE(mgr.SamplingRequired());
+  auto computed_period = mgr.NextSamplingTime() - px::chrono::coarse_steady_clock::now();
+  EXPECT_LE(computed_period, std::chrono::milliseconds{10000});
+  EXPECT_GE(computed_period, std::chrono::milliseconds{9990});
+}
+
+// Tests that the push period were set correctly for SamplePushFrequencyManager.
+TEST(SamplePushFrequencyManagerTest, SetPushPeriodEndAndCheck) {
+  SamplePushFrequencyManager mgr;
+  mgr.set_push_period(std::chrono::milliseconds{10000});
+  EXPECT_EQ(mgr.push_period(), std::chrono::milliseconds{10000});
+  EXPECT_TRUE(mgr.PushRequired(0.5, 0));
+
+  mgr.Push();
+  EXPECT_FALSE(mgr.PushRequired(0.5, 0));
+  auto computed_period = mgr.NextPushTime() - px::chrono::coarse_steady_clock::now();
+  EXPECT_LE(computed_period, std::chrono::milliseconds{10000});
+  EXPECT_GE(computed_period, std::chrono::milliseconds{9990});
+}
+
 }  // namespace stirling
 }  // namespace px